Controlling the release of such active ingredients as pharmaceuticals, fragrances, insecticides, and other agricultural chemicals has proven to be extremely useful for providing valuable advantages over conventional products. Such advantages include longer durations of efficacy, decreased toxicity, and more flexibility and convenience in handling and application. Control of the release of active ingredients has been achieved by diffusion through membranes, diffusion through pores, osmotic pumping, erodible polymers, mechanical fracture, electrical energy, and other means appropriate for specific applications.
For applications in which it is desirable or necessary for the active ingredient to be sprayed or distributed in particulate or liquid-suspension form, controlled-release microcapsules, granules, and porous particles have been developed. Many different types of microencapsulation processes have been developed, as described for example in Nixon, "Microencapsulation" (1976). Microencapsulation, however, involves polymerizing or solidifying a polymeric shell around a core of the active ingredient, and the active ingredient is therefore present during the entire process. Although this is not a problem with many active ingredients, some are degraded or otherwise affected by the processing conditions, and not all of the active ingredient is microencapsulated, due to solubility in the liquid phases present or to inefficiency in the process itself. Due to interactions between the active ingredient and the polymer, solvent, or other chemicals, the range of active ingredients, as well as polymer, is limited. In addition, solvents, reactants, or other ingredients used in microencapsulation processes may remain in the microcapsules with the active ingredient, which may cause toxicity or further degradation problems. Washing the microcapsules to remove contaminants is usually not practical, because some of the active ingredient will be lost as well. Additional problems with microcapsules include a limitation on size (usually to smaller than about 200 microns in diameter), a limitation on strength, and a limitation on content of active ingredient, especially in combination with high strength.
Porous granules or particles to provide controlled release are also known. Such porous particles can be made in the absence of active ingredient, thus eliminating problems related to interactions between the active ingredient and particle materials, and enabling the particles to be extracted or cleaned, if necessary, prior to loading with active ingredient. However, unless the pore sizes are small, the active ingredient is typically released too rapidly to be useful. In addition, it is typically released by desorption, evaporation, or by leaching, all of which are dependent on the environment of use. For example, in U.S. Pat. No. 4,111,684, there are disclosed porous cellulosic particles subsequently loaded with herbicides that must be water-leachable.
Other examples of microporous cellulosic particles are disclosed in U.S. Pat. Nos. 3,985,298 and 3,846,404. These particles shrink upon drying, and must therefore be loaded by a complicated process of diffusive exchange of the water initially present with the active ingredient desired, taking care to avoid drying. In a further example of microporous particles of controlled release, U.S. Pat. No. 3,639,306 discloses the preparation of active ingredient-encapsulating particles having a finely porous skin on the surface and a substantially hollow interior containing the active ingredient. Although such active ingredient-encapsulating particles are stated to be useful for slow release of active ingredient, due to depletion of active ingredient in the reservoir over time and the consequent decrease in active ingredient-releasing surface area, the rate of release of active ingredient decreases with time, rather than being constant.
In summary, there is a need in the art for high strength sprayable granules or particles that can be prepared independently of loading with active ingredient, and that can release active ingredient at a constant rate over a prolonged period of time.